1. Technical Field
The disclosed subject matter relates to a system for reducing an audible pop-up noise produced when a speaker is powered up.
2. Description of the Related Art
Generally, a mobile phone has different types of speakers, such as a high power speaker, earphone speakers and a handset speaker, in order to provide enhanced audio experiences to a user. These speakers are coupled through a pin interface to an integrated circuit included in the mobile phone. The integrated circuit incorporates a number of driver circuits, which are powered-up to drive these speakers during operation of the mobile phone.
In certain conditions, such as when connecting a headset with the mobile phone, an audible pop-up noise emanates from the speakers before an expected audio signal is received. The pop-up noise is produced due to mismatches between the driver circuits. These mismatches may be caused due to various factors such as a difference in transient responses of the driver circuits and an error (offset) between settled output values of the driver circuits coupled across a particular speaker.
Ideally, the driver circuits coupled to a differentially driven speaker should have the same transient response so as to provide a symmetrical settled voltage across the speaker. But, generally, there exists a difference in the transient responses of the driver circuits due to differences in rate of charging of the driver circuits. Such differences in charging rates cause a large differential voltage to form at the output of the driver circuits, and in turn across a differentially coupled speaker. This differential voltage activates the coupled speaker and results in a pop-up noise from the speaker.
Further, when an input signal is applied to the driver circuit, either as a normal signal or as a differential signal, a DC offset is observed at the pin interface. The DC offset can be suppressed by calibrating the driver circuit using a variety of techniques, for example, by using successive approximation registers. However, these calibration techniques employ a large number of measurements of the output signal at the pin interface where the calibration is to be applied. As a result, the calibration time of each of the driver circuits is very high.
Additionally, in case the input signal is applied to the driver circuits across different speakers, for example, to provide stereo playback, the DC offset appearing at the output of only one driver circuit can be corrected since the signal pathway is mono. As a result, the DC offset across the driver circuit of the other speaker causes an audible pop-up noise. Such an audible pop-up noise results in a substandard interface between the mobile phone and a user and therefore, degrades the perceived quality of operation of the mobile phone.